1. Field of the Invention
The present invention generally relates to a method of manufacturing a thin film transistor. More particularly, the present invention relates to a method of manufacturing a low temperature polysilicon thin film transistor (LTPS-TFT).
2. Description of Related Art
The traditional fabrication method of polysilicon thin film transistors includes a solid phase crystallization (SPC) process. Due to its high process temperature which is more than 1000° C., the SPC process uses a quartz substrate with high melting point. The quartz substrate is more expensive than the glass substrate. In addition, the panel size is merely about 2 to 3 inches because of the restriction of the substrate size. With the advance of laser technology, an excimer laser annealing process has been applied to LTPS-TFT processes.
The excimer laser annealing process applies a laser beam on the amorphous silicon layer, melting the structure thereof and then recrystallizing such amorphous silicon layer into a polysilicon layer. More important, the process can be finished under 600° C. The cheap glass substrate can also be applied to the fabrication of the polysilicon thin film transistor so as to create big size panel. The polysilicon thin film transistor generated from the process is also called a low temperature polysilicon thin film transistor.
In the LTPS-TFT processes, ion implantation processes are important. However, the gate insulating layer above the channel is usually damaged during the channel doping step so that the device characteristic, such as threshold voltage, is influenced. If the channel doping step is not carried out, the threshold voltage of NMOS transistor is not easily controlled. In addition, during source/drain ion implantation photoresist layers may be carbonized by the high-energy ion implantation so that the photoresist layers are not easily removed subsequently.